Positron emission tomography (PET) is a widely used medical imaging modality. PET scanning methods may be used to obtain an image of a subject, for example an image of a patient. In medical PET scanning, a radionuclide is introduced into the body of a patient and concentrates in an area of interest of the patient (for example, a tumour). The radionuclide emits positrons, which annihilate with electrons to produce pairs of back-to-back photons, each photon having an energy of 511 keV.
The patient or other subject is placed in a PET detector, which uses a plurality of detector elements to detect the emitted photons. The detector elements may be arranged as a ring-shaped array around the patient and may comprise, for example, scintillator crystals with associated photomultiplier tubes.
When a photon deposits energy in one of the detector elements, the PET system records when and where the energy deposit occurs, and may record how much energy was deposited. If a photon deposits all of its energy in a single detector element, an energy of around 511 keV will be recorded. Known PET systems use events in which energy depositions of around 511 keV are recorded in a pair of detector elements at opposite sides of the PET detector within a given time window (for example, 10 ns). Such events may be assumed to correspond to a pair of back-to-back photons from a positron annihilation. The system determines a line of response between the pair of detectors and assumes that the annihilation event occurred on that line of response.
However, some emitted photons undergo scattering before reaching a detector element. For example, emitted photons may undergo scattering in the body of the patient or other subject. Scattered photons may change direction on scattering.
Therefore, if one or both photons in a back-to-back pair is scattered before being received by a detector element, a line drawn between the detector elements at which the pair of photons are received may no longer pass through the location of the annihilation event. Such events may be called scattering events and the process for correcting for such events in data processing may be called scatter correction.
Furthermore, in some cases two annihilations occur very close in time and a first photon from one back-to-back pair may be incorrectly matched with a second photon from another back-to-back pair. Such events may be called random coincidences. Random events may also occur if a scintillator crystal self-emits light.
In some events, all of the energy of each incident photon is deposited in a single detector element, for example a single scintillator crystal. In other events, a photon undergoes Compton scattering within a first detector element, depositing part of its energy in that first detector element, and then deposits the remainder of its energy in one or more further detector elements. Events in which both of the photons undergo scattering in the detector (and therefore each photon deposits its energy in two or more detector elements) may be referred to as paired Compton or PC PET events.
Both scattering events and random coincidences add noise to the imaging data. A number of methods have been used to attempt to correct for scattering events and random coincidences in PET imaging data. For example, by setting a threshold close to 511 keV, many scattering events may be removed from the data (because energy is lost when the photon is scattered). However, if the threshold is set too close to 511 keV then a number of true events may also be removed. The typical energy resolution of the photon scintillator detectors employed in PET machines may limit the achievable separation of true and scattered events.
In some existing systems, a Monte Carlo simulation method is used to attempt to correct for scattering events. A first image is reconstructed using all the events received by the PET detector in which two energy depositions of around 511 keV were recorded within the same time window. This image may include scattering events, and so may be noisy. In typical PET imaging in three dimensions, the scattered photons may comprise 30% to 50% of the measured data.
A Monte Carlo simulation is then used to construct a scattering model that best matches the observed data. The Monte Carlo simulation models the distribution of mass inside the patient that is most likely to have resulted in the observed events. The resulting scattering model may be used to weight the event data, for example to weight events that are determined by the model to be scattering events out of the image to produce scatter-corrected data. An image may then be reconstructed from the scatter-corrected data. Scatter modelling and scatter compensation techniques are discussed in Habib Zaidi and Kenneth F. Koral, Scatter Modelling and Compensation in Emission Tomography, Eur J Nucl Med Mol Imaging 2004 May; 31(4); 761-82.
In some current systems, the construction of the scattering model may take some hours and may require considerable computing power. It may not be possible to construct the scattering model in real time, for example in the same session in which the PET data is acquired. For large imaging sources, PET systems may employ an additional scan with an X-ray source to map the source distribution. Performing an additional scan with an X-ray source may add dose to the scan.
It has previously been suggested that it may be possible to distinguish scattered events from unscattered events if polarisation information were available for each incident photon, for example if a polarimeter were used to measure the polarisation of each photon that is incident upon a detector (see, for example, McNamara et al, Positron emission tomography coincidence detection with photon polarization correction, Medical Imaging 2013: Physics of Medical Imaging, Proc of SPIE Vol 8668, 86681). McNamara et al simulated back to back photon pairs with orthogonal polarisation and used the simulation to extract the scalar product of the polarisation vectors of the two detected photons when they reach the detector. Current systems may not be capable of measuring the polarisation of the two photons event-by event.
Some PET data reconstruction methods have been proposed in which a Compton camera detector is used, the Compton camera comprising a scatterer in front of the scintillator crystals (see, for example, WO 2006/058432). A Compton camera may determine a cone describing a possible gamma source location for each event. The intersection of many ellipses may be used to determine the source location.